1. Field of the Invention
This invention relates to an alarm buzzer for producing a buzzing sound in an emergency such as a fire and burgulary.
2. Prior Art
One conventional alarm buzzer 100 (FIGS. 1 and 2) as disclosed in U.S. Pat. No. Re. 31,152 is of the type designed to be operated by DC power source and comprises a cup-shaped housing 102 of electrically conductive material, a dish-shaped horn 104 attached to an open end of the housing 102, a vibratory plate 105 of electrically-conductive material clamped between the housing 102 and the horn 104, and an electromagnet 106 mounted within the housing 102. An insulating plate 108 is placed on the bottom of the housing 102, and a yoke 110 is mounted on the insulating plate 108, the yoke 110 having a base portion 110a held against the insulating plate 108 and a pair of opposed inverted L-shaped arms 110b. A bobbin 112 of an electrically-insulating material is mounted on the yoke 110, the bobbin 112 having a base portion 112a held against the base portion 110a of the yoke 110 and a hollow cylindrical portion 112b formed on the base portion 112a and having a flange 112c formed on the free end of the cylindrical portion 112b . A coil 116 is wound around the cylindrical portion 112b, and an iron core 118 is inserted in the tubular portion 112b. The upper legs of the opposed arms 110b directed toward each other are held against the flange 112c of the bobbin 112. The yoke 110 has a mounting portion 110c extending perpendicularly from the base portion 110a, and an armature 120 is mounted on the mounting portion 110c through a leaf spring 122, the armature 120 being disposed in opposed relation to the upper end of the core 118 and the upper leges of the arms 110b. A pair of tubular projections 112d are formed on the base portion 112a of the bobbin 112, and pass through the base portion 110a of the yoke 110 and the insulating plate 108. A first lug terminal 124 is held in contact with the outer surface of the bottom wall of the housing 102 by a rivet 126 passing through the tubular projection 112d and the bottom wall of the housing 102. Another rivet 128 passes through the other tubular projection 112d. Thus, the bobbin 102, the yoke 110 and the insulating plate 108 are fixed with respect to the housing 102 by the two rivets 126 and 128. And, the yoke 110 is electrically insulated from the housing 102. A tubular projection 112e is also formed on the base portion 112a of the bobbin 112 and passes through the base portion 110a of the yoke 110, the insulating plate 108 and the bottom wall of the housing 102. A terminal 130 in the form of a strip extends through the tubular projection 112e. One end of the terminal 130 disposed in the housing 102 is electrically connected to one end of the coil 116, the other end of the coil 116 being electrically connected to the yoke 110. The terminals 124 and 130 are electrically connected to a DC power source. A first contact 132 is secured to the armature 120, and a second contact 134 is formed on the vibratory plate 105. When an electrical potential is applied across the terminals 124 and 130 from the DC power source, the electromagnet 106 consisting of the coil 116 and the core 118 is energized so that the armature 120 is attracted by the core 118 against the bias of the leaf spring 122 to bring the first contact 132 out of contact with the second contact 134 to de-energized the electromagnet 106 whereupon the armature 120 is brought into striking engagement with the vibratory plate 105 under the restoring force of the spring 122. This operation is repeated to produce a buzzing sound.
It has now been desired to provide an alarm buzzer analogous to the above-mentioned conventional buzzer but capable of being operated by an AC power source.